What Phil says is true, at least of the X-Men characters.
But I think they would bring in X-people; they've already had to compromise on Quicksilver and The Scarlet Witch. Corporate might not bother, but the creatives would probably care about, say, having The Beast in The Avengers.

However, I think Avengers 4 will be too soon for the merger; they won't have had the rights fully worked out by the time of release.

Honestly, I find myself not caring too much about mutants in the MCU.

I do expect an MCU Fantastic Four movie, though. And that I care about.

~ Le Messor
"Success is not forever and failure is not fatal."
~ Don Shula